Automatic volume control



V May 19, 1936. K. WILHELM 2,040,967

AUTOMATIC VOLUME CONTROL Filed June 28, 1935 fiff/F/[R AHJMPZ/F/fl? id i 5/50 52 .omwM/a 1 /510 1000 (O/Z SPEAAH? INVENTOR KARL WILHELM ATTORNEY Patented May 19, 1936 UNITED STATES PATENT OFFICE AUTOMATIC VOLUME CONTROL Germany Application iIune 28, 1933, Serial No. 677,998 In Germany August 26, 1932 3 Claims.

The present invention relates to amplifiers with automatic sensitivity regulation, especially radio receiver sets.

The operation of fading regulator means in a broadcast receiver set is liable to be greatly impaired by undesirable direct current couplings. To explain the situation reference may be made to the circuit diagram, in the drawing, comprising automatic volume control. The filament of the radio frequency amplifier tube H is connected with a potential which is approximately equal to that of the plate of the rectifier tube G. In order that a negative biasing voltage of the radio frequency amplifier tube H in the state of rest may be insured, the filament is maintained at a suitable potential by the agency of a voltage divider RI, R2. If a radio frequency voltage falls upon the grid of the plate rectifier tube, this causes a decrease in the voltage at the rectifier. As a result, the potential difference between grid and filament of the radio amplifier tubes is raised, indeed, a marked negative biasing voltage will be impressed thereon. Apart from that, however, undesirable action will arise, for at the same time there occurs a change in'the direct current of the radio amplifier tubes in that it decreases as the radio frequency amplitude grows, and a decrease happens also in the potential of the filament. In other words, this effect counteracts the regulation.

On the other hand, as a consequence of the diminution of the direct current of the amplifier tube, also the grid biasing voltage of the regulator rectifier tube decreases, and this means an incidental reduction in the plate voltage of this tube and an increase in the grid shift of the radio tubes. It will thus be noted that incidental secondary actions are liable to increase or decrease the desired useful action. Of course, the said secondary actions may be utilized. However, as a general rule it is preferable to preclude them completely, if possible.

The invention will now be explained. The receiving system shown in the drawing comprises a radio frequency amplifier H, a signal rectifier G, and an audio amplifier whose input electrodes are coupled to the output circuit of the rectifier. The dynamic loud speaker D has its moving coil connected into the output circuit of the audio amplifier, while the speaker field coil is denoted in two sections R1 and R2. These sections are connected across the direct voltage supply source, the plate of amplifier H being connected to the positive side of coil R1. The cathode of tube H is connected to a point intermediate coils R1 and R2, and the plate of rectifier G is also connected to the positive side of coil R1. The automatic volume control action is accomplished by connecting the control grid of amplifier H to the plate of rectifier G. It will be seen that, as the signal input to rectifier G increases, the negative bias on the control grid of amplifier H increases, and the gain of the amplifier is therefore decreased. Generally speaking, in every source of voltage whose terminal potential is to be stable in the presence of load fluctuations, the internal resistance of the source must be chosen as low as possible. What is imperative on this ground is the use of low ohm potentiometers RI, R2, although this is contrary to sound economy. In an outfit with field excitation for a dynamic loudspeaker D, however, this winding RI--R2 may be employed to act as a potentiometer.

Another means adapted to minimize the internal resistance resides in the use and provision of means designed to keep the current, or the voltage, at a constant and stable value such as glowdischarge voltage dividers; iron resistors confined inside a hydrogen atmosphere, etc.

What is claimed is:

1. In a radio receiver including a signal amplifier tube and signal rectifier, an audio amplifier having a loud speaker in its output circuit, a. field coil for the speaker, a source of direct voltage for the circuits of said receiver, said field coil being connected across said source, a connection between the anodes of said signal amplifier and rectifier and the positive side of said field coil, a connection between the cathode of said signal amplifier and an intermediate point on said field coil, and a direct current connection between the signal grid of the signal amplifier and the anode of said rectifier and the resistance of said coil being sufficiently low to maintain the voltage of said intermediate point substantially unaffected by variations in space current flow of said signal amplifier.

2. In a radio receiver including a signal amplifier tube and signal rectifier, an audio amplifier having a loud speaker in its output circuit, a field coil for the speaker, a source of direct voltage for the circuits of said receiver, said field coil being connected across said source, a connection between the anodes of said signal amplifier and rectifier and the positive side of said field coil, a connection between the cathode of said signal amplifier and an intermediate point on said field coil, a, direct current connection between the signal grid of the signal amplifier and the anode of said rectifier, and the resistance of said'coil being 55 'sufficien-tly low to maintain the voltage of said intermediate point substantially unaffected by variations in' space current flow of said signal amplifier and a connection between the cathode of said rectifier and the negative side of said field coil.

3. In a radio receiver including a. signal amplifier tube and signal rectifier, an audio amplifier having'a loud speaker in its output circuit, a field coil for the speaker, a source of direct voltage for the circuits of said receiver, said field'coil 'being connected across said'sour'ce, a connection between the anodes of said signal amplifier and rectifier and the positive side of said field coil, a

amplifier, and the resistance of, said coil being sufiiciently low to maintain theivoltage'of said' intermediate point substantially unafiected by variations in space current flow of said signal amplifier and the cathode of said rectifier being connected to a point of said field coil which is negative with respect to said intermediate point.

KARL WILHELM. 

